These were really quick shader development test renders, using the lighting set up from Project #1 and the Stanford Buddha and Stanford Lucy models.
Shot #1: Lucy Angel Statue and Buddha statue for comparison
Shot #2 : Shader development tests
Here I was trying to achieve the same light purple tone as the outer surface of my rock.
Shot #3: Shader Development Tests, Interior and Exterior Shaders
For this test, I duplicated the sphere, scaled it down, and applied a new shader to it so that I could start to achieve the idea of geometry within the sphere.
Shot #4: Refraction values on initial shader
Index of Refraction is set to 1.54, roughly the equivalent of quartz. Most polished/tumbled rocks from museums are quartz or a similar material.
I also found with my current lighting set up that the lighter the refraction color was, the more accurate my overall shader seemed to look. Darker refraction colors ended up rendering way too dark and it was hard to capture a look of transparency.